In grinding machines, and particularly grinders for finishing bearing races such as tapered roller bearing cones, it is frequently necessary to form a "crown" on the bearing truck, so that under subsequent loading, the crown or raised portion of the track will deflect, tending to create a straight raceway. Since the amount of crowning is often in the nature of approximately 50 millionths of an inch across a one inch wide track, for example, it can be appreciated that the radius of curvature of any portion of the crown is substantially great. Because the height of the crown is so small, it is not necessary that the crowned length be a perfect radius, but rather, it is sufficient if the crowned track is merely some curvilinear surface which has the requisite amount of center-of-track crown, terminating at the track ends. In the process of rough-turning of bearings, a relief portion is very often provided at the ends of the track to provide a relief area for an amount of grinding wheel which will slightly overhang the ends of the track.
Because of the great radius of curvature of the crowned track, it can be appreciated that a radial swinging mechanism would be impractical to apply to a grinder from an actual swing point. Several novel prior art devices, however, have been effectively used in creating the crown. One type of prior art device simply drags a follower across a curved surface and, as the follower is attached to a dependent dressing stylus, the stylus will trace out the same curvature on a dressing wheel. This type of dressing device has several disadvantages, in that the dressing cam can wear, and is therefore incapable of consistently dressing a constant shape on the wheel. Further, the crown shape is difficult to purposely vary, since it is not generated, and additional cams would be required.
A second prior art device involves a novel arrangement for generating a locus of curvature on a grinding wheel, wherein an angled cam bar is slid across a pair of simple supports in a direction substantially parallel to a wheel face desired to be crowned. The cam bar has a pivot point which traces out a locus of curvature as the cam bar attitude varies during cam bar traversal across the fixed simple supports. A dressing stylus depends from the pivot point and thereby traces out the identical generated locus of curvature on a grinding wheel. The radius of curvature tends to be less at the midspan point between the simple supports, and the locus of curvature becomes flatter, with an increased radius of curvature as the pivot point approaches each support. This second prior art device has lent itself well to providing a crown curvature to roller bearing tracks. However, a substantially sharp corner is created at the ground track ends as the flattened radius of curvature intercepts the relief portions in the bearing. It has been found through bearing research that the sharp corner created by the track end and the relief portion tends to break down after a period of use, and further, that if the track portion is blended by a smooth radius into the relief portion at the ends, that bearing life may be substantially increased.
No such prior art devices as previously described, contain a means for generating a substantially sharper radius of curvature at the ends of a substantially large locus of curvature. Applicant has obviated the problems inherent in the prior art devices, by a novel device for dressing a grinding wheel, which incorporates substantially similar parameters as the aforementioned cam bar dressing device, wherein a first portion of a locus of curvature is generated in a substantially similar manner to the prior art device, but a predetermined ends of the first portion of the locus of curvature, the cam bar support is moved in such fashion as to create a sharper radius of curvature in the locus of curvature as the cam bar continues traversal.
It is therefore an object of the present invention to provide a wheel dressing device for generating a locus of curvature on a dressing wheel having a first portion of substantially large radii of curvature, and second and third portions contiguous to said first portion, wherein said second and third portions are of substantially reduced radii of curvature.